Soundproofing panel construction

ABSTRACT

A soundproofing panel particularly suited for use in a sound control cabinet for machinery. In its preferred form, the panel comprises in respective laminations a relatively rigid, soundadmitting, perforate wall formed of a matrix of large apertures for mechanical strength and a matrix of small apertures for blocking penetration by small or relatively pointed objects, an acoustically transparent membrane for excluding foreign material, a void layer of sound-absorbing material, and an acoustically limp, imperforate wall of high mass-per-unit area for preventing transmission of residual sound through the imperforate wall from the sound-absorbing material.

SOUNDPROOFING PANEL CONSTRUCTION [76] Inventor: William 0. Evans, Jr., 218 E.

Streetsboro St., Hudson, Ohio 44236 [22] Filed: Sept. 6, 1973 [21] Appl. No.: 394,833

[52] US. Cl. 181/33 K; 181/33 G; 161/41 [51] Int. Cl E04b H86 [58] Field of Search 181/33 G, 33 GA, 33 GB,

[56] References Cited UNITED STATES PATENTS 2,809,145 10/1957 McDermott 181/33 G 2,998,337 8/1961 Tillotson 181/33 G 3,021,914 2/1962 Wilson 181/33 G 3,166,149 l/1965 Hulse et al... 181/33 G 3,502,171 3/1970 Cowan 181/33 G 3,509,963 5/1970 DePlante 181/33 G 3,573,145 3/1971 Witkoski et a1 161/41 3,592,288 8/1971 Walter 181/33 G 3,622,430 11/1971 .lurisich 161/68 3,630,312 12/1971 Woodward et a1. 181/42 3,738,900 6/1973 Matzke 161/116 X [451 May 6,1975

OTHER PUBLICATIONS US. Pat. No. 3,770,560, Nov. 6, 1973, filed 10/21/71, Elder et al.,

Primary ExaminerRichard B. Wilkinson Assistant Examiner-Vit W. Miska Attorney, Agent, or FirmMcNenny, Farrington, Pearne & Gordon [5 7] ABSTRACT A soundproofing panel particularly suited for use in a sound control cabinet for machinery. In its preferred form, the panel comprises in respective laminations a relatively rigid, sound-admitting, perforate wall formed of a matrix of large apertures for mechanical strength and a matrix of small apertures for blocking penetration by small or relatively pointed objects, an acoustically transparent membrane for excluding foreign material, a void layer of sound-absorbing material, and an acoustically limp, imperforate wall of high mass-per-unit area for preventing transmission of residual sound through the imperforate wall from the sound-absorbing material.

19 Claims, 4 Drawing Figures SOUNDPROOFING PANEL CONSTRUCTION BACKGROUND OF THE INVENTION The invention relates to sound-absorbing panels and. more specifically, to a panel construction particularly adapted for control of machinery-generated noise in industrial and commercial environments.

DESCRIPTION OF THE PRIOR ART Medical and social awareness of the permanent. destructive effect of noise on human hearing has resulted in growing legislation and social pressure directed to abatement of excessive machine-based noise. Technical and economic factors often favor arrangements for absorbing or muffling machine-generated noise rather than or in combination with efforts to eliminate the actual noise sources.

A known technique for controlling excessive noise is the provision of an acoustic cover or jacket over a machine or critical portion of a machine to absorb the noise generated by it. The interior of the enclosure is thus acoustically isolated from the surrounding personnel space. Various panel constructions have been proposed for use in these applications, such as illustrated in U.S. Pat. Nos. 3,322,233 and 3,478,958.

Besides providing adequate sound control, it is important that an acoustic panel be suitable for its intended environment. Prior panel arrangements have exhibited varying degrees of sound-absorbing effectiveness but have not always been equal to the rigors of industrial and commercial service. Panels have often been susceptible to both immediate and cumulative loss of efficiency as the result of exposure to dirt-laden atmospheres and to fluids, such as water or water vapor. lubricants, detergents, and other chemicals, which in one manner or another come into contact with the sound-absorbing material of the panel and, by saturating it, greatly diminish its performance.

SUMMARY OF THE INVENTION The invention provides an integrated, self-supporting panel structure having a plurality of laminations each producing an acoustic or structural function, or both. The elements comprising the various panel layers. and their respective arrangement, provide an, efficient combination for maximum sound control and structural integrity. The acoustic panel, in accordance with the invention, comprises a pair of spaced walls, one perforate and the other imperforate, and an intermediate body of sound energy-absorbing material. When assembled as a machine enclosure, the perforate wall faces the interior of the enclosure or cabinet, while the imperforate wall provides the exterior cabinet surface.

The inner perforate wall permits substantially free passage of sound into the sound-absorbing medium, contributes a large portion of the stiffness of the selfsupporting panel, and protects the interior of the panel from physical damage. The imperforate wall, normally forming the visible exterior of a cabinet, is an acoustically limp"structure characterized by a relatively high sound transmission loss factor such that it reflects a substantial portion of the sound energy which reaches it back into the sound-absorbing medium. Such a wall should have a sound transmission class value in the range from about 20 to about 40. The sound energyabsorbing material sandwiched between the perforate wall on one side of the panel and the imperforate wall on the other side of the panel should be a high-void material having a noise reduction coefficient (NRC) in the range of about 0.70 to about 0.90.

In the preferred embodiment, the perforate wall includes two superimposed matrices each presenting a different aperture size. The first matrix is preferably a sheet of expanded sheet metal having relatively large apertures and of sufficient gauge and body to provide requisite panel stiffness and physical protection for the panel interior. The second matrix, interior of the expanded metal matrix, is formed of wire cloth having an opening size substantially less than that of the expanded metal matrix. The wire cloth is adapted to complement the protection afforded by the expanded metal by blocking the latters larger apertures to objects of measurable size, while maintaining sufficient open area for passage of sound into the sound-absorbing material.

The interior of the panel contains sound-absorbing material, typically a hollow or unfilled mass of cellular. fibrous, or granular structure, such as sound-absorbing polyurethane foam. A sound-transmitting membrane, ideally a film of chemically inert plastic film, is positioned between the perforate wall and the soundabsorbing material. The membrane is acoustically transparent but prevents entrance of solid and fluid contaminants into the sound-absorbing material, which, by its unfilled nature, is generally pervious to such contaminants.

The imperforate outer wall has a relatively high mass and low stiffness to provide a sound-reflecting layer by which substantially more energy is reflected back into the sound absorber than is transmitted into surrounding space. In the disclosed embodiment, the imperforate wall is composed of an inner sheet of lead and an adjacent outer sheet of high density plastic material that is relatively stiff compared to the lead sheet for supporting and protecting the lead sheet against physical damage, but soft enough to have substantial soundreflecting properties. Thus, the sheets of lead and plastic material, together, provide a high mass for the imperforate wall, while the plastic sheet additionally provides exterior protection against physical damage and an attractive appearance at the exterior surface of the panel, and may be colored and/or decoratively embossed for the latter purpose. A laminated panel constructed in accordance with the invention provides a high degree of sound attenuation within practical limits of overall weight and material cost, while providing a high resistance to physical and chemical damage, an attractive appearance, and good cleanability of both interior and exterior surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a soundproofing cabinet assembly for isolating and absorbing the noise produced by an enclosed machine;

FIG. 2 is a perspective view of a typical selfsupporting panel constructed in accordance with the invention;

FIG. 3 is a perspective view, on an enlarged scale, of a fragmentary section of the panel with various portions of its elements broken away to reveal its constructional details; and

FIG. 4 is a fragmentary, cross sectional view, on an enlarged scale. of a panel taken along the line 44 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Noise generated by operation of machinery, such as a screw machine or other machine tool, may be absorbed and isolated from a room or other area in which it is operated by enclosing it with a soundproof shell or cabinet, generally indicated at 11. As illustrated. the cabinet assembly 11 comprises a rectangular, rigid, skeletal frame 12 on which are mounted a plurality of generally rectangular, planar panels 13. The frame 12 is formed of lengths of angle iron 16 or other structural elements, welded or otherwise secured into a threedimensional, rectangular form large enough to surround the machine 10. As in most instances, the cabinet 11 does not include a separate horizontal floor panel, but is arranged with its side panels closely adjacent the floor supporting the machine 10 to prevent escape of noise energy from the cabinet interior.

At least several of the panels 13a may be movably mounted on the frame 12, such as by hinges 17, for access to the machine 10 for adjustment, repair, etc. The specific details of the frame 12, its configuration, and the manner of mounting the panels 13 thereon form no part of the present invention except for generally illustrating one manner of arranging the panels 13 in relationship to a machine tool.

' As shown in FIGS. 2 and 4, each panel 13 ideally includes a perimeter frame 19 of U cross section. The periineter frame 19 is formed of aluminum or other structural metal, and may be fabricated by welding or otherwise securing appropriate lengths of U stock at its corners 21. The frame 19 contributes to the rigidity of the panel 13 and sandwiches or holds together various layers or laminations of the panel described below. Each panel 13 is preferably self-supporting, either in a vertical or a horizontal orientation, so that the panels 13 require no support at their midportions or within the boundaries of the perimeter frame 19. The panels 13 consequently may be mounted on the cabinet frame 12 by suitable fasteners (not shown) without additional bridging to otherwise prevent the panels from excessive flexing or sagging.

Referring now in particular to FIGS. 3 and 4, the main body of the panel 13 comprises a plurality of coextensive layers in the form of a perforate wall 26, a sound-absorbing layer or medium 27, and an imperforate wall 28. With the panels 13 forming a machine enclosure such as the cabinet II, the perforate wall 26 faces the interior of the cabinet. Ideally, the perforate wall 26 comprises two matrices, here represented by separate sheets 31 and 32, defining apertures of substantially different size. The outer sheet 31, in the illustrated embodiment, is formed of expanded, flattened sheet metal, with an open area of, for example, 60 percent of its total area. The inner perforate sheet 32 is formed of an open weave fabric, such as wire cloth commonly used for window screening. Apertures 33 of the expanded metal 31 are preferably in the range of /2 by 1 inch, for example, while the screen is about 16 mesh per inch, so that the difference in aperture size between these sheets is one or more orders of magnitude. The open area of the screen 32 should not be less than 50 percent, and is preferably in the range of 60 to 70 percent so that the total effective open area of the perforate wall 26 is not less than percent and, preferably, is around 35 percent or more of its total area.

The sound-absorbing layer 27 is relatively thick in comparison with the remaining layers of the panel 13,

and comprises a low density void material usually either cellular, fibrous, or granular in structure. Such material, as is understood by those skilled in the art, absorbs sound energy passing into it by converting it to heat. Ideally, the material 27 is a semi-rigid foam hav ing sufficient compressive strength to maintain the opposed walls 26 and 28 apart at least against their own weight when disposed in a horizontal orientation without excessive compression of its overall thickness.

A membrane 36 of chemically inert material, such as an inert plastic film, is disposed between the soundabsorbing material 27 and the perforate wall 26. The membrane 36 may have a thickness in the range of 0.5 to 4 mils, but is preferably about 1.5 mils thick. Owing to its negligible mass, this membrane is acoustically transparent and thereby transmits substantially all of the sound energy impinging on it, with negligible reflection. The membrane 36 prevents entrance of foreign material normally encountered in machinery operations, particularly fluids such as water, water vapor, lubricating greases and oils, detergents and chemical cleaners, into the sound-absorbing material. Permeation of such materials into the sound-absorbing medium 27, by filling its voids, greatly diminishes its 7 sound-absorbing characteristics.

As illustrated, the outer imperforate wall 28 includes two separate sheets or layers 38 and 39 of acoustically limp material. The inner sheet 38 is formed of extremely high density, low rigidity material, such as lead sheet stock having a density of approximately 1 pound per square foot and an stc factor of about 22 to 23. The outer sheet 39 is formed of a high density plastic, and preferably has an stc factor of about 15 or more. In combination, the lead and plastic sheets 38 and 39 form a layer at which substantially more sound energy is reflected back into the sound-absorbing material 27 than is transmitted through the layer. The zone or wall 28, owing to its relatively high-mass-per-unit area and limited stiffness, as compared to a plain metal wall of structural metal such as steel or aluminum, forms a high sound transmission loss wall having an stc factor of about 30.

I have found that, in the interest of economy and simplicity of construction, the sheet lead component of the imperforate wall 28 may be replaced by a high mass, vinyl-base sheet made from a vinyl molding resin loaded with a dense, acoustically limp filler, such as powdered lead, barium sulfate, or the like. Various high-mass plastic sheet materials are available having stc factors in the range of about 20 to about 30, in thicknesses of about one-eighth to three-sixteenths inch weighing about 1 pound per square foot. A onestructure, i.e., relatively high mass and relatively low stiffness, so that a diaphragm or drum effect is not produced wherein the wall, by its vibration, would become a secondary source of sound radiation to the outside of I the panel. In general, the sound transmission loss of the wall 28 is proportional to its mass and inversely proportional to its rigidity.

The expanded metal sheet 31 of the perforate wall 26, according to the invention. provides a substantial portion of the overall panel rigidity. The perforate wall 26 thus provides the two important functions of protecting the interior of the panel and providing stiffness to the panel without contributing to noise control problems by producing a diaphragm or drum effect at the exterior of the cabinet 11. The assembled panel 13 is thus characterized by a pair of spaced walls in which the ratio of stiffness to mass-per-unit area for the inner wall 26 is substantially greater than the corresponding ratio of stiffness to mass-per-unit area at the outer wall. It is contemplated that other equivalent imperforate wall structures may be employed in place of the twolayer wall 28, e.g., a single sheet of plastic filled with lead or other high density material in particulate or pellet form.

By way of example. for panels ranging up to about 6 feet square and a permissible thickness to about 1% inches, the following construction may be used: Expanded sheet metal (31) Aluminum, 0.051 inch gauge, V2 X l-inch openings,

and about 60% open area Screen (32) Aluminum window screen having about 16 mesh- /inch and about 65% open area Membrane (36) 1.5 mil Tedlar" (a polyvinyl fluoride film manufactured by E. l. DuPont de Nemours and Company) Sound-absorbing medium (27) Polyurethane. acoustical foam, 60 ppi (pores/inch) lnner high transmission loss layer (38) Sheet lead, about 1 pound/sq. foot Outer high transmission loss layer (39) One-eighth inch Noryl" high density plastic sheet,

about 0.75 pounds/sq. foot (a polyphenylene oxide sheet material manufactured by General Electric Co.)

The bight section. designated 41, of the perimeter channel frame 19, is dimensioned with respect to the total thickness of the panel layers to provide approximately one-eighth inch compression of the soundabsorbing foam 27 along the edges thereof to maintain the panel layers firmly together in the assembly.

The above-disclosed panel 13 is particularly suited for covering machinery. since it is generally not susceptible to damage caused by ordinary physical or chemical abuse. The perforate wall 26, while allowing suffimaterial 27, provides protection against perforation of both the membrane 36 and sound-absorbing material. The outer expanded metal matrix 31 affords protection against inadvertent high energy blows from relatively oils, and the like, from the sound-absorbing material 27, and permits the expanded metal 31 and mesh 32 to be periodically washed with detergents or chemical 'cient passage area for noise into the sound-absorbing cleaners without these fluids likewise contaminating the sound-absorbing material.

Although a preferred embodiment of the invention is illustrated, it is to be understood that various modifications and rearrangements of parts may be resorted to without departing from the scope of the invention disclosed and claimed herein.

What is claimed is:

l. A soundproofing panel for covering machinery comprising, in successive laminations, a relatively rigid perforate wall, a thin sound-transmitting membrane, a relatively thick layer of low density void soundabsorbing material, and a layer of acoustically limp material of substantially greater density than said void sound-absorbing material, said membrane being formed of an imperforate, nonporous material that is chemically inert and substantially unaffected by fluids normally encountered in machinery environments, said perforate wall having an effective open area for sound transmission to said sound-absorbing material of at least 30 percent of its total area and having a mechanical strength sufficient to protect the membrane from puncture when struck by inadvertent blows from rigid objects, said membrane preventing passage of machinery fluids through said perforate wall into said void material and thereby avoiding contamination and loss of the sound-absorbing performance of said material.

2. A panel as set forth in claim 1, wherein said perforate wall comprises a first sheet having relatively small apertures and a second sheet having relatively large apertures.

3. A panel as set forth in claim 2, wherein said sheet of small apertures is arranged between said sheet of large apertures and said membrane.

4. A soundproofing panel for covering machinery comprising a substantially rigid perforate wall and a layer of void relatively non-rigid sound-absorbing material adjacent to and substantially coextensive with said perforate wall, said perforate wall having an open area of at least 30 percent of its total area, said perforate wall having a set of large openings therein at a first plane and a set of small openings at a second plane, said large openings having dimensions of the same order of magnitude as the thickness of the panel, said small openings having dimensions of at least one order of magnitude smaller than the thickness of the panel. a sound-transmitting membrane between said perforate wall and said sound-absorbing material, said membrane being formed of a material substantially impervious to fluids normally found in machinery operations.

5. A panel as set forth in claim 4, wherein said plane of small openings is between said plane of large openings and said sound-absorbing material.

6. A panel as set forth in claim 4, wherein said rigid wall comprises a pair of separately formed apertured sheets, one of said sheets providing said large openings and the other sheet providing the small openings.

7. A panel as set forth in claim 6, wherein said other sheet is disposed between said one sheet and said sound-absorbing material.

8. A panel as set forth in claim 7, wherein said one sheet comprises expanded sheet metal stock.

9. A panel as set forth in claim 8, wherein said other sheet comprises wire fabric.

10. A panel as set forth in claim 9, wherein said sheets are both formed of aluminum.

11. A panel as set forth in claim 4, wherein said panel I includes an acoustically limp imperforate wall on a side of said sound-absorbing material opposite said rigid perforate wall.

12. A soundproofing panel comprising in successive, substantially coextensive laminations, a perforate rigid wall having an effective open area of at least 30 percent of its total area to reduce its acoustical reflecting and dampening properties, a layer of void sound-absorbing material, and an imperforate high acousitcal transmission loss wall, said imperforate wall including nonstructural high density material of the order of at least 1 pound per square foot, said perforate wall having substantially less mass-per-unit area than said imperforate wall, while providing substantially more rigidity to the panel per-unit-mass than said perforate wall, a soundtransmitting imperforate membrane adjacent said perforate wall to prevent entrance of foreign material into said sound-absorbing material, said membrane being substantially chemically inert whereby it is impervious to fluids normally encountered in machinery operations.

13. A panel as set forth in claim 12, wherein said membrane is disposed between said perforate wall and said sound-absorbing material, whereby said membrane is protected from puncture by said perforate wall.

14. A panel as set forth in claim 13, wherein said perforate wall comprises a firstmatrix having apertures of relatively large size and a second matrix superimposed over the apertures of the first matrix and having apertures of relatively small size. the first matrix providing the major portion of the rigidity of the perforate wall 15. A panel as set forth in claim 14, wherein said matrices are arranged in separate planes.

16. A panel as set forth in claim 15, wherein the plane of the second matrix is between the plane of the first matrix and said membrane.

17. In combination, a rectangular rigid framework adapted to be positioned over a machine. a plurality of self-supporting soundproofing panels on the frameand said sound-absorbing material, an acoustically transparent, substantially chemically inert membrane between said other lamina and said sound-absorbing material, said membrane preventing entrance of foreign material into said sound-absorbing material, said perforate wall protecting said membrane from physical damage, said imperforate wall being acoustically limp as a result of a relatively high mass-per-unit area and relatively low rigidity, the stiffness to mass-per-unit area ratio of said perforate wall being substantially greater than the corresponding ratio of stiffness to mass-per-unit area of said imperforate wall.

18. The combination of claim 17 in which said imperforate outer wall comprises two separately formed imperforate laminae, one being a relatively thin inner limina of sheet lead and the other being a relatively thicker. non-metallic, outer lamina of substantially greater stiffness than the sheet lead for supporting and protecting the sheet lead against physical damage.

19. The combination of claim 17 in which said imperforate outer wall comprises two separately formed imperforate laminae, one being a relatively thin inner lamina of sheet lead and the other being a relatively thicker, outer lamina of substantially greater stiffness than the sheet lead for supporting and protecting the sheet lead against physical damage.- 

1. A soundproofing panel for covering machinery comprising, in successive laminations, a relatively rigid perforate wall, a thin sound-transmitting membrane, a relatively thick layer of low density void sound-absorbing material, and a layer of acoustically limp material of substantially greater density than said void sound-absorbing material, said membrane being formed of an imperforate, nonporous material that is chemically inert and substantially unaffected by fluids normally encountered in machinery environments, said perforate wall having an effective open area for sound transmission to said sound-absorbing material of at least 30 percent of its total area and having a mechanical strength sufficient to protect the membrane from puncture when struck by inadvertent blows from rigid objects, said membrane preventing passage of machinery fluids through said perforate wall into said void material and thereby avoiding contamination and loss of the sound-absorbing performance of said material.
 2. A panel as set forth in claim 1, wherEin said perforate wall comprises a first sheet having relatively small apertures and a second sheet having relatively large apertures.
 3. A panel as set forth in claim 2, wherein said sheet of small apertures is arranged between said sheet of large apertures and said membrane.
 4. A soundproofing panel for covering machinery comprising a substantially rigid perforate wall and a layer of void relatively non-rigid sound-absorbing material adjacent to and substantially coextensive with said perforate wall, said perforate wall having an open area of at least 30 percent of its total area, said perforate wall having a set of large openings therein at a first plane and a set of small openings at a second plane, said large openings having dimensions of the same order of magnitude as the thickness of the panel, said small openings having dimensions of at least one order of magnitude smaller than the thickness of the panel, a sound-transmitting membrane between said perforate wall and said sound-absorbing material, said membrane being formed of a material substantially impervious to fluids normally found in machinery operations.
 5. A panel as set forth in claim 4, wherein said plane of small openings is between said plane of large openings and said sound-absorbing material.
 6. A panel as set forth in claim 4, wherein said rigid wall comprises a pair of separately formed apertured sheets, one of said sheets providing said large openings and the other sheet providing the small openings.
 7. A panel as set forth in claim 6, wherein said other sheet is disposed between said one sheet and said sound-absorbing material.
 8. A panel as set forth in claim 7, wherein said one sheet comprises expanded sheet metal stock.
 9. A panel as set forth in claim 8, wherein said other sheet comprises wire fabric.
 10. A panel as set forth in claim 9, wherein said sheets are both formed of aluminum.
 11. A panel as set forth in claim 4, wherein said panel includes an acoustically limp imperforate wall on a side of said sound-absorbing material opposite said rigid perforate wall.
 12. A soundproofing panel comprising in successive, substantially coextensive laminations, a perforate rigid wall having an effective open area of at least 30 percent of its total area to reduce its acoustical reflecting and dampening properties, a layer of void sound-absorbing material, and an imperforate high acousitcal transmission loss wall, said imperforate wall including nonstructural high density material of the order of at least 1 pound per square foot, said perforate wall having substantially less mass-per-unit area than said imperforate wall, while providing substantially more rigidity to the panel per-unit-mass than said perforate wall, a sound-transmitting imperforate membrane adjacent said perforate wall to prevent entrance of foreign material into said sound-absorbing material, said membrane being substantially chemically inert whereby it is impervious to fluids normally encountered in machinery operations.
 13. A panel as set forth in claim 12, wherein said membrane is disposed between said perforate wall and said sound-absorbing material, whereby said membrane is protected from puncture by said perforate wall.
 14. A panel as set forth in claim 13, wherein said perforate wall comprises a first matrix having apertures of relatively large size and a second matrix superimposed over the apertures of the first matrix and having apertures of relatively small size, the first matrix providing the major portion of the rigidity of the perforate wall.
 15. A panel as set forth in claim 14, wherein said matrices are arranged in separate planes.
 16. A panel as set forth in claim 15, wherein the plane of the second matrix is between the plane of the first matrix and said membrane.
 17. In combination, a rectangular rigid framework adapted to be positioned over a machine, a plurality of self-supporting soundproofing panels on the framework, said panels including An inner perforate wall, an imperforate outer wall spaced from the inner perforate wall, a relatively thick layer of low density void sound-absorbing material between said walls, said perforate wall being formed of two separately formed perforate laminae, one of said perforate laminae being relatively rigid and having relatively large apertures, the other of said perforate laminae having relatively small apertures and being disposed between said one perforate lamina and said sound-absorbing material, an acoustically transparent, substantially chemically inert membrane between said other lamina and said sound-absorbing material, said membrane preventing entrance of foreign material into said sound-absorbing material, said perforate wall protecting said membrane from physical damage, said imperforate wall being acoustically limp as a result of a relatively high mass-per-unit area and relatively low rigidity, the stiffness to mass-per-unit area ratio of said perforate wall being substantially greater than the corresponding ratio of stiffness to mass-per-unit area of said imperforate wall.
 18. The combination of claim 17 in which said imperforate outer wall comprises two separately formed imperforate laminae, one being a relatively thin inner limina of sheet lead and the other being a relatively thicker, non-metallic, outer lamina of substantially greater stiffness than the sheet lead for supporting and protecting the sheet lead against physical damage.
 19. The combination of claim 17 in which said imperforate outer wall comprises two separately formed imperforate laminae, one being a relatively thin inner lamina of sheet lead and the other being a relatively thicker, outer lamina of substantially greater stiffness than the sheet lead for supporting and protecting the sheet lead against physical damage. 